Low-energy electronic structure in Y1-xCaxBa2Cu3O7-y comparison of t ime-resolved optical spectroscopy, NMR, neutron and tunneling data

TL;DR

This study compares various spectroscopic techniques to understand the pseudogap and superconducting gap evolution in Y1-xCaxBa2Cu3O7-y, revealing different gap types and a crossover to BCS-like behavior near optimal doping.

Contribution

It introduces a unified energy level diagram explaining different gap measurements and their relation to local pairing and superconductivity in cuprates.

Findings

Different spectroscopies observe distinct gaps: spin gap and charge gap.

A crossover to BCS-like superconductivity occurs near optimal doping.

The proposed model links local triplet pairs to the spin gap and dissociation energy to the charge gap.

Abstract

Time-resolved optical measurements give information on the quasiparticle relaxation dynamics in YBCO, from which the evolution of the gap with doping and temperature can be systematically deduced. In this paper these optical charge-channel `pseudogap' data are compared with the `pseudogap' obtained from the NMR Knight shift Ks, spin polarized neutron scattering (SPNS) and single particle tunneling measurements. A simple energy level diagram is proposed to explain the different `gap' magnitudes observed by different spectroscopies in Y1-xCaxBa2Cu3O7-y, whereby the spin gap Delta_s in NMR and SPNS corresponds to a triplet local pair state, while Delta_p in the charge excitation spectrum corresponds to the pair dissociation energy. At optimum doping and in the overdoped state, an additional T-dependent gap becomes evident, which closes at T_c, suggesting a cross-over to a more conventional BCS-like superconductivity scenario.